Electric discharge device



April 18, 1939. L, PERRGTT 2,155,237

ELECTRIC DISCHARGE DEVICE Filed Nov. 18, 1937 m B R R R A/VOE cu/mavr64mm.) M ii 0 m a0- so a0 a0 ma //0 T/ME (sew/v05) Inventor Lawren ce FTPerrotc His Attorhev.

Patented Apr. 18, 1939 V ATENT OFFIQE ELECTRIC DISCHARGE DEVICE LaurenceF. Perrott, Revere, Mass., assignor to General Electric Company, acorporation of New York Application November 18, 1937, Serial No.175,183

3 Claims.

The present invention relates to electric discharge devices and moreparticularly to improvements in cathodes for such devices. While notlimited thereto, the invention is especially applicable to dischargedevices of the type which employ a readily ionizable medium such asmercury vapor in the discharge space.

Cathodes comprising alkaline earth materials are highly efiicient inoperation where efficiency is measured in terms of emission current perwatt of heating energy supplied to the cathode. However, due to thevaporization characteristics of the alkaline earth metals it isimpracticable to operate such cathodes at temperatures appreciably above950 C. At this temperature the emission current which can be derivedfrom a unit of cathode area is so small that a relatively extendedcathode surface must be provided if considerable total emission isdesired.

While the use of an extended surface cathode is in no way objectionableafter normal operation is established, it involves the disadvantage ofprolonging unduly the time required to heat the cathode to an effectiveemitting temperature during initial starting periods. For manyapplications where instantaneous availability of the discharge device isdesirable this is a highly objectionable feature.

It is an object of the present invention to provide a cathode which isas highly efiicient in normal operation yet which is capable of assumingfull load within at least a few seconds after its initial energization.

It is a further object of the invention to provide a quick-actingcathode which is characterized by a long commercially useful life.

According to a preferred embodiment of the invention the foregoingobjects are accomplished by providing a composite cathode combining anextended surface having an alkaline earth coating with a resistanceheater which is of low thermal inertia and which also has an electronemitting portion capable of carrying the full load current of the devicefor at least a substantial period of time. Preferably the cathode partsare so constructed that while the discharge current may initially becarried by the resistance heater alone, it is eventually transferred tothe alkaline earth coated surfaces as normal operating conditions areattained.

The features of novelty which I desire to protect herein are pointed outwith particularity in the appended claims. The invention itself,together with further objects and advantages thereon may best beunderstood by reference to the following description taken in connectionWith the drawing in which Fig. 1 represents a discharge device suitablyembodying my improved form of cathode; Fig. 2 comprises a detailed viewin partial section of a cathode constructed in accordance with theinvention; Fig. 3 is a plan view looking down on the top of theconstruction of Fig. 2; and Fig. 4 is a graphical representation usefulin explaining the operation of my improved cathode.

Referring particularly to Fig. 1 I have shown a discharge devicecomprisinga sealed envelope ID, for example, of glass, enclosingdischarge electrodes and a quantity of a readily ionizable mediumadapted to facilitate the maintenance of an arc-like discharge.comprise, for example, vaporizable substance such as mercury, a fixedgas such as argon, or the combination of a vaporizable substance and afixed gas.

The electrode structure comprises an anode ll, suitably of graphite, anda composite cathode comprising a hollow member l2 and a resistanceheater I3 for helping to maintain the cathode at a temperature ofeffective electron emission. Lead-in conductors l5, l6, and I1 aresealed through the walls of the envelope for energizing the anode andcathode and for supplying heating current to the heater [3 from asuitable potential source (not shown) connected between the conductorsl6 and IT. The details of the cathode construction are shown in Figs. 2and 3 in which parts illustrated in Fig. 1 are similarly numbered.

In accordance with the invention, one part of the cathode comprises amember or members providing relatively extended surfaces exposed to thedischarge space and having a coating of a material which has electronemission character'- istics comparable to those of alkaline earthmaterials. Such surfaces may be provided in various ways as by the useof vaned or corrugated structures or by utilizing a hollow enclosurehaving an opening of substantial area therein. In the cathodeillustrated in Fig. 2 the latter ar rangement is exemplified by theprovision of an elongated hollow metal cylinder 52, for example ofnickel, having an open end thereof directed toward the anode II. Inorder to increase its mechanical strength the cylinder I2 is provided ateach end with a circumferentially extending beading or corrugation 19.It is coated either :15 Such medium may interiorly or exteriorly or bothwith .an alkaline device the cylinder I2 is heated to a temperature ofeffective electron emission (about 950 C.) by the effects of the arcdrop and by the use of a resistance heater in good heat-exchangingrelation therewith. Such a heater is illustrated in the present case asa spirally coiled filament l3 having its axis substantially co-linearwith that of the cylinder l2, although it is evident that other formsmay be employed. For instance, the heater itself may be of complexconstruction embodying a resistance element and another elementindirectly heated thereby.

For the purposes of the present invention, it is desired that the heatercombine low thermal inertia with high electron emissivity. The formerquality is achieved by limiting the mass and area of the heater as bygiving it a filamentary form such as that shown. The quality of highemissivity is best attained by so proportioning the heater as to causeit to operate at a very high temperature (in response to the passage ofa-heating current therethrough) and by embodying in it an electronemissive material adapted to function satisfactorily at suchtemperature. Morespecifically, I prefer to employ for this purpose amaterial which is. capable of operating without excessive vaporizationat temperatures appreciably above the permissible operating range ofalkaline earth materials and which is capable at such temperatures ofdeveloping for at least substantial periods of time an electron emissioncurrent. equal to the full load cur-. rent of the discharge device.

A specific material which admirably fulfills the requirements set forthin the foregoing comprises tungsten, especially when used in connectionwith an activating coating such, for example,

as thorium oxide, 1. e. thoria. (A tungsten filament coated with thoriahas an emissivity at 2000 C. many times in excess of that of tungstenalone). I prefer to use as a heater body, a filament comprising atungsten mandrel wire overwound with a helix of finer tungsten wire, thewhole being coated with thoria and activated by heating in vacuum or inan inert gas at about 2800 C. In Figs. 2 and 3 a filament I3 of thistype is shown as being suspended by means of a conducting supportingmember 20 which is set into a conducting rod 2| on which the cylinder I2is mounted.

The operation of the invention may best be understood by referring againto Fig. l in which the cathode is assumed to be of a construction suchas'that described. If a discharge producing potential is impressedbetween the anode H and the cathode l2 simultaneously with the supplyingof heating current to the filament l3, the latter element will reach atemperature of effective electron emission very quickly. Due to the lowthermal inertia of the filament for example, a temperature of 2000 C.may be attained within at least a few seconds. Under these conditions,the discharge device may almost immediately pass full load current, suchcurrent being drawn from the exposed surfaces of the filament eventhough the cylinder. I2 is at too low a temperature to contributeappreciably to the discharge. However, as the cylinder gradually heatsup due to the generation of heat by the filament and byrthe arcdischarge, an increasing proportion of the discharge current will bediverted to the activated surfaces of the cylinder.

This diversion of current and its variation with time is illustrated inFig. 4 in which the curve A represents the portion of the dischargecurrent supplied by the filament l3 and the curve B represents thatportion supplied by the cylinder I2, these being plotted against elapsedtime following the initial energization of the device. Referring to thecurve A, it will be noted that the filament current rises almostimmediately (actually within about 1 seconds) to a value correspondingto the full load current of the discharge device. Thereafterthe filamentcurrent begins to decline as a result of the increased diversion ofcurrent to the more extended surfaces of the cylinder l2. After aboutsixty seconds, of the total current will be drawn from the cathodecylinder and only the residual 20% from the filament.

The fact that the discharge current is progressively diverted to thecathode cylinder and does not continue from the filament itself isexplainab-le in part by the relatively high arc drop required tomaintain adequate emission from the filament. Due to the smalltotal areaof the filaments emitting surfaces the field free emission of thefilament may be less than that required by the discharge. Consequentlythis deficiency in emission will be compensated by an increase in thecathode drop and a corresponding increase in the total are drop.However, as soon as the cathode cylinder reaches an effective emittingtemperature, the discharge current may be supplied from this source witha lower arc drop 1;

than is required to maintain the necessary emission from the filament.Consequently, the arc will seek the path of least resistance and will betransferred at least preponderantly to the cylinder.

It is important to note that the heating current supplied to thefilament I3 need not be in excess of that required to maintain thecylinder l2 at the desired temperature during the normal operatingperiod, taking into account the assistance;

mionic emission alone all the electron currenti required during theinitial starting period. The deficiency will be made up, however, byfield emission due to a temporary increase in the arc drop as previouslyexplained. Because of the exist-.

ence of this mechanism for temporarily enhancing the filament emission,the cathode as a whole may be operated at all times with a heatingenergy input no greater than that required to obtain effective emissionfrom the alkaline earth material. ciency of such material the cathodewill therefore be characterized by a high degree of efiiciency.(Measured in terms of emission current per watt of heating energysupplied.)

It will be understood that the ion bombardment which is incidental tothe high are drop obtaining during the initial starting period of thedevice would tend to destroy the filament if continued for a long periodof time. However, due to its relatively short duration its actualdestructive eifects are substantially negligible. Thus a filament whichmight be consumed in a relatively few hours if continuously subjected toion bombardment, will nevertheless survive many thous- Due to theinherently high emitting effiand starting operations under the specialconditions stated herein.

A further advantage which is attributable to the presence of a high aredrop during the initial starting period is the effect of thecorrespondingly high arc losses in decreasing the heating time of thecathode cylinder l2. It will be understood that during such startingperiod this cylinder receives heat not only by direct radiation from thefilament itself but also by radiation from the arc. The latter componentis substantially increased by the existence of a high are drop.

While I have referred particularly to the use of thoria as a preferredemitting material for use with a resistance heater it will be understoodthat other substances may be alternatively employed. For example, onemay use for this purpose thoriated tungsten providing a sufiicientlyhigh gas pressure is employed in the discharge device to preventstripping or de-activation of the filament material by excessivebombardment. One may also employ uranium oxide as a substitute forthorium oxide as a coating material. Furthermore, while I haveexemplified the invention by reference to a discharge device employing asingle anode, it is equally applicable to devices in which two or moreanodes are used. I aim, in the appended claims, to cover all suchmodifications of structure and use as come within the true spirit andscope of the foregoing disclosure.

What I claim as new and desire to obtain by Letters Patent of the UnitedStates is:

1. An electric discharge device comprising an envelope enclosing aquantity of a readily ionizab'le medium, an anode and a cathode of thetypewhich requires a continuous supply of heating current thereto, saidcathode including a relatively massive structure of high thermal inertiacoated with alkaline earth material and exposed to the discharge spaceand a quick-acting element comprising a heater of low thermal inertiaeffective to maintain the alkaline earth material at its proper emittingtemperature during normal operation of the device, the heater being alsoexposed to the discharge space and having a coating of a substancehaving substantially the emission characteristics of thoria whereby itis enabled to supply the full load current of the device during startingperiods without excessive vaporization.

2. An electric discharge device comprising an envelope enclosing aquantity of a readily ionizable medium, an anode, and a cathodeincluding a hollow metal structure of high thermal inertia coatedinteriorly with an alkaline earth material and a filamentary heatereffective by virtue of its heating properties to cause the said coatedstructure to develop substantially full load current during normaloperation of the device, the

said heater being exposed to the discharge space and being coated withthoria whereby it is enabled to carry the full load current of thedevice without excessive vaporization during initial operating periods.

3. A cathode comprising a hollow open-ended metal cylinder having acoating of alkaline earth material applied to the surfaces thereof and aheating filament of overwound tungsten arranged within the saidcylinder, the filament being coated with thoria whereby it is enabled todevelop the full rated emission current of the cathode during startingperiods and being proportioned to heat the said cylinder to atemperature of eifective electron emission during normal operation.

LAURENCE F. PERROTT.

